Procedures for managing secondary eNB (SeNB) radio link failure (S-RLF) in dual connectivity scenarios

ABSTRACT

Certain aspects of the present disclosure provide procedures for managing secondary eNB (SeNB) radio link failure (S-RLF) in dual connectivity scenarios. A user equipment (UE) may establish communication with a Master Evolved Node B (MeNB) and a Secondary eNB (SeNB). The UE may detect a Radio Link Failure (RLF) of a connection with the SeNB and may transmit an indication of the RLF to the MeNB, in response to the detection. The MeNB may take at least one action to manage the RLF, in response to receiving the indication of the RLF, for example, including transmitting a reconfiguration command to the UE. The SeNB may also detect the RLF and transmit an indication of the RLF to the MeNB over a backhaul connection, in response to the detection.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application for patent is a continuation of U.S. patentapplication Ser. No. 14/608,688 by Vajapeyam et al., filed Jan. 29,2015, which claims priority to U.S. Provisional Application No.61/934,258, filed Jan. 31, 2014, which are assigned to the assignee ofthe present application and expressly incorporated by reference hereinin its entirety.

FIELD

The present disclosure relates generally to managing secondary eNB(SeNB) radio link failure (S-RLF) in dual connectivity scenarios.

BACKGROUND

Wireless communication systems are widely deployed to provide varioustelecommunication services such as telephony, video, data, messaging,and broadcasts. Typical wireless communication systems may employmultiple-access technologies capable of supporting communication withmultiple users by sharing available system resources (e.g., bandwidth,transmit power). Examples of such multiple-access technologies includecode division multiple access (CDMA) systems, time division multipleaccess (TDMA) systems, frequency division multiple access (FDMA)systems, orthogonal frequency division multiple access (OFDMA) systems,single-carrier frequency division multiple access (SC-FDMA) systems, andtime division synchronous code division multiple access (TD-SCDMA)systems.

These multiple access technologies have been adopted in varioustelecommunication standards to provide a common protocol that enablesdifferent wireless devices to communicate on a municipal, national,regional, and even global level. An example of an emergingtelecommunication standard is Long Term Evolution (LTE). LTE is a set ofenhancements to the Universal Mobile Telecommunications System (UMTS)mobile standard promulgated by Third Generation Partnership Project(3GPP). It is designed to better support mobile broadband Internetaccess by improving spectral efficiency, lower costs, improve services,make use of new spectrum, and better integrate with other open standardsusing OFDMA on the downlink (DL), SC-FDMA on the uplink (UL), andmultiple-input multiple-output (MIMO) antenna technology. However, asthe demand for mobile broadband access continues to increase, thereexists a need for further improvements in LTE technology. Preferably,these improvements should be applicable to other multi-accesstechnologies and the telecommunication standards that employ thesetechnologies.

SUMMARY

Certain aspects of the present disclosure provide an apparatus forwireless communications implemented, for example, by User Equipment(UE). The apparatus generally includes at least one processor and amemory coupled to the at least one processor. The at least one processoris generally configured to establish communication with a Master EvolvedNode B (MeNB) and a Secondary eNB (SeNB), detect a Radio Link Failure(RLF) of a connection with the SeNB, and transmit an indication of theRLF to the MeNB, in response to the detection.

Certain aspects of the present disclosure provide an apparatus forwireless communications implemented, for example, by a Master enodeB(MeNB). The apparatus generally includes at least one processor and amemory coupled to the at least one processor. The at least one processoris generally configured to establish a first connection with a UserEquipment (UE), configure the UE to establish a second connection with aSecondary Evolved Node B (SeNB), receive an indication of a Radio LinkFailure (RLF) of the second connection, and take at least one action tomanage the RLF, in response to receiving the indication of the RLF,where one action may be reconfiguring the UE.

Certain aspects of the present disclosure provide an apparatus forwireless communications implemented, for example, by Secondary eNodeB(SeNB). The apparatus generally includes at least one processor and amemory coupled to the at least one processor. The at least one processoris generally configured to establish communication with a Master EvolvedNode B (MeNB) and a User Equipment (UE), detect a Radio Link Failure(RLF) of a connection with the UE, and transmit an indication of the RLFto the MeNB over a backhaul connection, in response to the detection.

Aspects of the present disclosure also provide various other apparatusesand program products capable of performing operations described below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an example of a network architecture.

FIG. 2 is a diagram illustrating an example of an access network.

FIG. 3 is a diagram illustrating an example of a DL frame structure inLTE.

FIG. 4 is a diagram illustrating an example of an UL frame structure inLTE.

FIG. 5 is a diagram illustrating an example of a radio protocolarchitecture for the user and control planes.

FIG. 6 is a diagram illustrating an example of an evolved Node B anduser equipment in an access network.

FIG. 7 illustrates a dual connectivity scenario in accordance withcertain aspects of the present disclosure.

FIG. 8 illustrates a dual connectivity scenario including a macro celland a small cell communicating with a UE, in accordance with certainaspects of the present disclosure.

FIG. 9 illustrates continuous carrier aggregation, in accordance withcertain aspects of the present disclosure.

FIG. 10 illustrates non-continuous carrier aggregation, in accordancewith certain aspects of the present disclosure.

FIG. 11 illustrates a method for controlling radio links in a multiplecarrier wireless communication system by grouping physical channels, inaccordance with certain aspects of the present disclosure.

FIG. 12 illustrates a U-plane architecture including eNB specificbearers, in accordance with certain aspects of the present disclosure.

FIG. 13 illustrates an alternative U-plane architecture including asplit bearer, in accordance with certain aspects of the presentdisclosure.

FIG. 14 illustrates example operations performed, for example, by a UE,in accordance with certain aspects of the present disclosure.

FIG. 15 illustrates example operations performed, for example, by aMeNB, in accordance with certain aspects of the present disclosure.

FIG. 16 illustrates example operations performed, for example, by aSeNB, in accordance with certain aspects of the present disclosure.

DETAILED DESCRIPTION

Presently, UEs receive data from one eNodeB. However, users on a celledge may experience high inter-cell interference which may limit thedata rates. Dual connectivity allows users to communicate with twoeNodeBs simultaneously by sending and receiving data from the twoeNodeBs in two totally separate streams. By scheduling two independentdata streams to the UE from the two different eNodeBs at the same time,dual connectivity exploits uneven loading.

In an aspect of the present disclosure, dual connectivity enables a UEto simultaneously connect to a Master eNB (MeNB) and a secondary eNB(SeNB) which may not be collocated and, thus, may be connected by anon-ideal backhaul. Under dual connectivity a UE may benefit fromcarrier-aggregation (CA) gains in heterogeneous deployments.

In certain aspects, due to the distributed nature of the dualconnectivity deployment scenario (e.g., separate eNBs connected via anon-ideal backhaul) separate uplink control channels for both eNBs (MeNBand SeNB) is used to support distributed scheduling and independent MAC(Medium Access Control) operation across the eNBs. In an aspect, aspecial cell on the SeNB, Primary Secondary Cell (SpCell), is introducedin order to support the uplink control channels for the SeNB. Thepresence of an uplink control channel for the SeNB motivates the needfor a special radio link failure (RLF) procedure for the SeNB. Thisspecial RLF procedure for the SeNB may be referred to as the S-RLF.

According to certain aspects of the present disclosure, an S-RLFprocedure may include a UE detecting RLF of a connection with the SeNBand transmitting an indication of the RLF to the MeNB, in response tothe detection. The MeNB may take at least one action to manage the RLF,in response to receiving the indication of the RLF, for example, bytransmitting a reconfiguration command to the UE. In an aspect, SeNB mayalso detect the RLF and transmit an indication of the RLF to the MeNBover a backhaul connection, in response to the detection.

The detailed description set forth below in connection with the appendeddrawings is intended as a description of various configurations and isnot intended to represent the only configurations in which the conceptsdescribed herein may be practiced. The detailed description includesspecific details for the purpose of providing a thorough understandingof various concepts. However, it will be apparent to those skilled inthe art that these concepts may be practiced without these specificdetails. In some instances, well known structures and components areshown in block diagram form in order to avoid obscuring such concepts.

Several aspects of telecommunication systems will now be presented withreference to various apparatus and methods. These apparatus and methodswill be described in the following detailed description and illustratedin the accompanying drawing by various blocks, modules, components,circuits, steps, processes, algorithms, etc. (collectively referred toas “elements”). These elements may be implemented using electronichardware, computer software, or any combination thereof. Whether suchelements are implemented as hardware or software depends upon theparticular application and design constraints imposed on the overallsystem.

By way of example, an element, or any portion of an element, or anycombination of elements may be implemented with a “processing system”that includes one or more processors. Examples of processors includemicroprocessors, microcontrollers, digital signal processors (DSPs),field programmable gate arrays (FPGAs), programmable logic devices(PLDs), state machines, gated logic, discrete hardware circuits, andother suitable hardware configured to perform the various functionalitydescribed throughout this disclosure. One or more processors in theprocessing system may execute software. Software shall be construedbroadly to mean instructions, instruction sets, code, code segments,program code, programs, subprograms, software modules, applications,software applications, software packages, routines, subroutines,objects, executables, threads of execution, procedures, functions, etc.,whether referred to as software, firmware, middleware, microcode,hardware description language, or otherwise. The software may reside ona computer-readable medium. The computer-readable medium may be anon-transitory computer-readable medium. A non-transitorycomputer-readable medium include, by way of example, a magnetic storagedevice (e.g., hard disk, floppy disk, magnetic strip), an optical disk(e.g., compact disk (CD), digital versatile disk (DVD)), a smart card, aflash memory device (e.g., card, stick, key drive), random access memory(RAM), read only memory (ROM), programmable ROM (PROM), erasable PROM(EPROM), electrically erasable PROM (EEPROM), a register, a removabledisk, and any other suitable medium for storing software and/orinstructions that may be accessed and read by a computer. Thecomputer-readable medium may be resident in the processing system,external to the processing system, or distributed across multipleentities including the processing system. The computer-readable mediummay be embodied in a computer-program product. By way of example, acomputer-program product may include a computer-readable medium inpackaging materials. Those skilled in the art will recognize how best toimplement the described functionality presented throughout thisdisclosure depending on the particular application and the overalldesign constraints imposed on the overall system.

Accordingly, in one or more exemplary embodiments, the functionsdescribed may be implemented in hardware, software, firmware, or anycombination thereof. If implemented in software, the functions may bestored on or encoded as one or more instructions or code on acomputer-readable medium. Computer-readable media includes computerstorage media. Storage media may be any available media that can beaccessed by a computer. By way of example, and not limitation, suchcomputer-readable media can comprise RAM, ROM, EEPROM, CD-ROM or otheroptical disk storage, magnetic disk storage or other magnetic storagedevices, or any other medium that can be used to carry or store desiredprogram code in the form of instructions or data structures and that canbe accessed by a computer. Disk and disc, as used herein, includescompact disc (CD), laser disc, optical disc, digital versatile disc(DVD), floppy disk and Blu-ray disc where disks usually reproduce datamagnetically, while discs reproduce data optically with lasers.Combinations of the above should also be included within the scope ofcomputer-readable media.

FIG. 1 is a diagram illustrating an LTE network architecture 100 inwhich aspects of the present disclosure may be practiced.

For example, as indicated above, a UE (e.g. UE 102) may be duallyconnected to a MeNB (e.g., eNodeB 106) and a SeNB (one of the other eNBs108) simultaneously. In an aspect, once UE 102 is connected to the MeNB106, the MeNB 106 may configure the UE to establish connection with theSeNB 108, including transmitting configuration parameters to the UE 102regarding S-RLF. In an aspect, the MeNB 106 and the SeNB 108 may beconnected by a non-ideal backhaul. As noted above, an S-RLF procedurefor the SeNB 108 may include S-RLF detection, S-RLF indication, and atleast one of S-RLF recovery or SeNB release including bearer fallbackupon S-RLF. In certain aspects, the UE 102 may detect RLF of aconnection with the SeNB 108 (S-RLF) and transmit an indication of theS-RLF to the MeNB 106. In an aspect, in response to detecting the S-RLF,the UE may also suspend communication with the SeNB to prevent uplinkinterference with uplink transmission of other UEs in the vicinity of UE102. In an aspect the UE 102 may provide the indication of the S-RLF tothe MeNB 106 by transmitting a Radio Link Control (RRC) message to theMeNB 106. Upon receiving the indication of S-RLF from the UE 102, MeNB106 may take at least one action to manage the S-RLF, includingtransmitting a reconfiguration command to the UE. In an aspect, thereconfiguration command may include at least one of SeNB release, SeNBadd (for adding another SeNB), SeNB modify (e.g., modify transmitpower), or data fallback. In certain aspects, the SeNB 108 may alsodetect RLF of a connection with the UE 102 and may transmit anindication of the RLF to the MeNB 106 over the backhaul connection, inresponse to the detection.

The LTE network architecture 100 may be referred to as an Evolved PacketSystem (EPS) 100. The EPS 100 may include one or more user equipment(UE) 102, an Evolved UMTS Terrestrial Radio Access Network (E-UTRAN)104, an Evolved Packet Core (EPC) 110, a Home Subscriber Server (HSS)120, and an Operator's IP Services 122. The EPS can interconnect withother access networks, but for simplicity those entities/interfaces arenot shown. As shown, the EPS provides packet-switched services, however,as those skilled in the art will readily appreciate, the variousconcepts presented throughout this disclosure may be extended tonetworks providing circuit-switched services.

The E-UTRAN includes the evolved Node B (eNB) 106 and other eNBs 108.The eNB 106 provides user and control plane protocol terminations towardthe UE 102. The eNB 106 may be connected to the other eNBs 108 via an X2interface (e.g., backhaul). The eNB 106 may also be referred to as abase station, a base transceiver station, a radio base station, a radiotransceiver, a transceiver function, a basic service set (BSS), anextended service set (ESS), or some other suitable terminology. The eNB106 provides an access point to the EPC 110 for a UE 102. Examples ofUEs 102 include a cellular phone, a smart phone, a session initiationprotocol (SIP) phone, a laptop, a personal digital assistant (PDA), asatellite radio, a global positioning system, a multimedia device, avideo device, a digital audio player (e.g., MP3 player), a camera, agame console, or any other similar functioning device. The UE 102 mayalso be referred to by those skilled in the art as a mobile station, asubscriber station, a mobile unit, a subscriber unit, a wireless unit, aremote unit, a mobile device, a wireless device, a wirelesscommunications device, a remote device, a mobile subscriber station, anaccess terminal, a mobile terminal, a wireless terminal, a remoteterminal, a handset, a user agent, a mobile client, a client, or someother suitable terminology.

The eNB 106 is connected by an S1 interface to the EPC 110. The S1bearer connects an eNB to the core network. The EPC 110 includes aMobility Management Entity (MME) 112, other MMEs 114, a Serving Gateway116, and a Packet Data Network (PDN) Gateway 118. The MME 112 is thecontrol node that processes the signaling between the UE 102 and the EPC110. Generally, the MME 112 provides bearer and connection management.All user IP packets are transferred through the Serving Gateway 116,which itself is connected to the PDN Gateway 118. The PDN Gateway 118provides UE IP address allocation as well as other functions. The PDNGateway 118 is connected to the Operator's IP Services 122. TheOperator's IP Services 122 may include the Internet, the Intranet, an IPMultimedia Subsystem (IMS), and a PS Streaming Service (PSS).

FIG. 2 is a diagram illustrating an example of an access network 200 inan LTE network architecture in which aspects of the present disclosuremay be practiced.

In certain cases, a UE 206 that is at the cell edge of a cell 202 maynot efficiently communicate on the UL with its serving eNB 204 due topower limitations, UL interference etc. As shown in FIG. 3, UE 206 maybe at overlapping cell edges of cell 202 served by macro eNB 204 (e.g.,MeNB) and cell 210 served by a lower power class eNB 208 (e.g., SeNB),and may be dually connected to both eNB 204 and eNB 208 simultaneously.In an aspect, eNB 204 and eNB 208 may be connected by a non-idealbackhaul. In certain aspect, UE 206 may detect RLF of a connection withthe eNB 208 (S-RLF) and indicate the S-RLF to the eNB 204, for example,by transmitting a RRC message. Concurrently, the UE 206 may also suspendall communication with eNB 208 to prevent uplink interference withuplink transmission of other UEs in the vicinity of UE 206. Uponreceiving the indication of S-RLF from the UE 206, eNB 204 may take atleast one action to manage the S-RLF, including transmitting areconfiguration command to the UE 206. In certain aspects, the eNB 208may also detect RLF of a connection with the UE 206 and may transmit anindication of the RLF to the eNB 204 over the backhaul connection, inresponse to the detection.

In this example, the access network 200 is divided into a number ofcellular regions (cells) 202. One or more lower power class eNBs 208 mayhave cellular regions 210 that overlap with one or more of the cells202. A lower power class eNB 208 may be referred to as a remote radiohead (RRH). The lower power class eNB 208 may be a femto cell (e.g.,home eNB (HeNB)), pico cell, or micro cell. The macro eNBs 204 are eachassigned to a respective cell 202 and are configured to provide anaccess point to the EPC 110 for all the UEs 206 in the cells 202. Thereis no centralized controller in this example of an access network 200,but a centralized controller may be used in alternative configurations.The eNBs 204 are responsible for all radio related functions includingradio bearer control, admission control, mobility control, scheduling,security, and connectivity to the serving gateway 116.

The modulation and multiple access scheme employed by the access network200 may vary depending on the particular telecommunications standardbeing deployed. In LTE applications, OFDM is used on the DL and SC-FDMAis used on the UL to support both frequency division duplexing (FDD) andtime division duplexing (TDD). As those skilled in the art will readilyappreciate from the detailed description to follow, the various conceptspresented herein are well suited for LTE applications. However, theseconcepts may be readily extended to other telecommunication standardsemploying other modulation and multiple access techniques. By way ofexample, these concepts may be extended to Evolution-Data Optimized(EV-DO) or Ultra Mobile Broadband (UMB). EV-DO and UMB are air interfacestandards promulgated by the 3rd Generation Partnership Project 2(3GPP2) as part of the CDMA2000 family of standards and employs CDMA toprovide broadband Internet access to mobile stations. These concepts mayalso be extended to Universal Terrestrial Radio Access (UTRA) employingWideband-CDMA (W-CDMA) and other variants of CDMA, such as TD-SCDMA;Global System for Mobile Communications (GSM) employing TDMA; andEvolved UTRA (E-UTRA), Ultra Mobile Broadband (UMB), IEEE 802.11(Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, and Flash-OFDM employingOFDMA. UTRA, E-UTRA, UMTS, LTE and GSM are described in documents fromthe 3GPP organization. CDMA2000 and UMB are described in documents fromthe 3GPP2 organization. The actual wireless communication standard andthe multiple access technology employed will depend on the specificapplication and the overall design constraints imposed on the system.

The eNBs 204 may have multiple antennas supporting MIMO technology. Theuse of MIMO technology enables the eNBs 204 to exploit the spatialdomain to support spatial multiplexing, beamforming, and transmitdiversity. Spatial multiplexing may be used to transmit differentstreams of data simultaneously on the same frequency. The data steamsmay be transmitted to a single UE 206 to increase the data rate or tomultiple UEs 206 to increase the overall system capacity. This isachieved by spatially precoding each data stream (i.e., applying ascaling of an amplitude and a phase) and then transmitting eachspatially precoded stream through multiple transmit antennas on the DL.The spatially precoded data streams arrive at the UE(s) 206 withdifferent spatial signatures, which enables each of the UE(s) 206 torecover the one or more data streams destined for that UE 206. On theUL, each UE 206 transmits a spatially precoded data stream, whichenables the eNB 204 to identify the source of each spatially precodeddata stream.

Spatial multiplexing is generally used when channel conditions are good.When channel conditions are less favorable, beamforming may be used tofocus the transmission energy in one or more directions. This may beachieved by spatially precoding the data for transmission throughmultiple antennas. To achieve good coverage at the edges of the cell, asingle stream beamforming transmission may be used in combination withtransmit diversity.

In the detailed description that follows for some examples, variousaspects of an access network will be described with reference to a MIMOsystem supporting OFDM on the DL. OFDM is a spread-spectrum techniquethat modulates data over a number of subcarriers within an OFDM symbol.The subcarriers are spaced apart at precise frequencies. The spacingprovides “orthogonality” that enables a receiver to recover the datafrom the subcarriers. In the time domain, a guard interval (e.g., cyclicprefix) may be added to each OFDM symbol to combat inter-OFDM-symbolinterference. The UL may use SC-FDMA in the form of a DFT-spread OFDMsignal to compensate for high peak-to-average power ratio (PAPR).

FIG. 3 is a diagram 300 illustrating an example of a DL frame structurein LTE. A frame (10 ms) may be divided into 10 equally sized sub-frames.Each sub-frame may include two consecutive time slots. A resource gridmay be used to represent two time slots, each time slot including aresource block. The resource grid is divided into multiple resourceelements. In LTE, a resource block contains 12 consecutive subcarriersin the frequency domain and, for a normal cyclic prefix in each OFDMsymbol, 7 consecutive OFDM symbols in the time domain, or 84 resourceelements. For an extended cyclic prefix, a resource block contains 6consecutive OFDM symbols in the time domain and has 72 resourceelements. Some of the resource elements, as indicated as R 302, 304,include DL reference signals (DL-RS). The DL-RS include Cell-specific RS(CRS) (also sometimes called common RS) 302 and UE-specific RS (UE-RS)304. UE-RS 304 are transmitted only on the resource blocks upon whichthe corresponding physical DL shared channel (PDSCH) is mapped. Thenumber of bits carried by each resource element depends on themodulation scheme. Thus, the more resource blocks that a UE receives andthe higher the modulation scheme, the higher the data rate for the UE.

FIG. 4 is a diagram 400 illustrating an example of an UL frame structurein LTE. The available resource blocks for the UL may be partitioned intoa data section and a control section. The control section may be formedat the two edges of the system bandwidth and may have a configurablesize. The resource blocks in the control section may be assigned to UEsfor transmission of control information. The data section may includeall resource blocks not included in the control section. The UL framestructure results in the data section including contiguous subcarriers,which may allow a single UE to be assigned all of the contiguoussubcarriers in the data section.

A UE may be assigned resource blocks 410 a, 410 b in the control sectionto transmit control information to an eNB. The UE may also be assignedresource blocks 420 a, 420 b in the data section to transmit data to theeNB. The UE may transmit control information in a physical UL controlchannel (PUCCH) on the assigned resource blocks in the control section.The UE may transmit only data or both data and control information in aphysical UL shared channel (PUSCH) on the assigned resource blocks inthe data section. A UL transmission may span both slots of a subframeand may hop across frequency.

A set of resource blocks may be used to perform initial system accessand achieve UL synchronization in a physical random access channel(PRACH) 430. The PRACH 430 carries a random sequence and cannot carryany UL data/signaling. Each random access preamble occupies a bandwidthcorresponding to six consecutive resource blocks. The starting frequencyis specified by the network. That is, the transmission of the randomaccess preamble is restricted to certain time and frequency resources.There is no frequency hopping for the PRACH. The PRACH attempt iscarried in a single subframe (1 ms) or in a sequence of few contiguoussubframes and a UE can make only a single PRACH attempt per frame (10ms).

FIG. 5 is a diagram 500 illustrating an example of a radio protocolarchitecture for the user and control planes in LTE. The radio protocolarchitecture for the UE and the eNB is shown with three layers: Layer 1,Layer 2, and Layer 3. Layer 1 (L1 layer) is the lowest layer andimplements various physical layer signal processing functions. The L1layer will be referred to herein as the physical layer 506. Layer 2 (L2layer) 508 is above the physical layer 506 and is responsible for thelink between the UE and eNB over the physical layer 506.

In the user plane, the L2 layer 508 includes a media access control(MAC) sublayer 510, a radio link control (RLC) sublayer 512, and apacket data convergence protocol (PDCP) 514 sublayer, which areterminated at the eNB on the network side. Although not shown, the UEmay have several upper layers above the L2 layer 508 including a networklayer (e.g., IP layer) that is terminated at the PDN gateway 118 on thenetwork side, and an application layer that is terminated at the otherend of the connection (e.g., far end UE, server, etc.).

The PDCP sublayer 514 provides multiplexing between different radiobearers and logical channels. The PDCP sublayer 514 also provides headercompression for upper layer data packets to reduce radio transmissionoverhead, security by ciphering the data packets, and handover supportfor UEs between eNBs. The RLC sublayer 512 provides segmentation andreassembly of upper layer data packets, retransmission of lost datapackets, and reordering of data packets to compensate for out-of-orderreception due to hybrid automatic repeat request (HARQ). The MACsublayer 510 provides multiplexing between logical and transportchannels. The MAC sublayer 510 is also responsible for allocating thevarious radio resources (e.g., resource blocks) in one cell among theUEs. The MAC sublayer 510 is also responsible for HARQ operations.

In the control plane, the radio protocol architecture for the UE and eNBis substantially the same for the physical layer 506 and the L2 layer508 with the exception that there is no header compression function forthe control plane. The control plane also includes a radio resourcecontrol (RRC) sublayer 516 in Layer 3 (L3 layer). The RRC sublayer 516is responsible for obtaining radio resources (i.e., radio bearers) andfor configuring the lower layers using RRC signaling between the eNB andthe UE.

FIG. 6 is a block diagram of an eNB 610 in communication with a UE 650in an access network, in which aspects of the present disclosure may bepracticed.

For example, a UE (e.g. UE 650) may be dually connected to a MeNB (e.g.,eNodeB 610) and another SeNB (not shown) simultaneously. The MeNB 610and the SeNB may be connected by a non-ideal backhaul. In certainaspects, the UE 650 may detect RLF of a connection with the SeNB (S-RLF)and transmit an indication of the S-RLF to the MeNB 610. Additionally,in response to detecting the S-RLF, the UE 650 may suspend allcommunication with the SeNB. Upon receiving the indication of S-RLF fromthe UE 650, MeNB 610 may take at least one action to manage the S-RLF,including transmitting a reconfiguration command to the UE 650. In anaspect, the reconfiguration command may include at least one of SeNBrelease, SeNB add (for adding another SeNB), SeNB modify (e.g., modifytransmit power), or data fallback. In certain aspects, the SeNB also maydetect RLF of a connection with the UE 650 and may transmit anindication of the RLF to the MeNB 610 over the backhaul connection, inresponse to the detection.

In the DL, upper layer packets from the core network are provided to acontroller/processor 675. The controller/processor 675 implements thefunctionality of the L2 layer. In the DL, the controller/processor 675provides header compression, ciphering, packet segmentation andreordering, multiplexing between logical and transport channels, andradio resource allocations to the UE 650 based on various prioritymetrics. The controller/processor 675 is also responsible for HARQoperations, retransmission of lost packets, and signaling to the UE 650.

The TX processor 616 implements various signal processing functions forthe L1 layer (i.e., physical layer). The signal processing functionsincludes coding and interleaving to facilitate forward error correction(FEC) at the UE 650 and mapping to signal constellations based onvarious modulation schemes (e.g., binary phase-shift keying (BPSK),quadrature phase-shift keying (QPSK), M-phase-shift keying (M-PSK),M-quadrature amplitude modulation (M-QAM)). The coded and modulatedsymbols are then split into parallel streams. Each stream is then mappedto an OFDM subcarrier, multiplexed with a reference signal (e.g., pilot)in the time and/or frequency domain, and then combined together using anInverse Fast Fourier Transform (IFFT) to produce a physical channelcarrying a time domain OFDM symbol stream. The OFDM stream is spatiallyprecoded to produce multiple spatial streams. Channel estimates from achannel estimator 674 may be used to determine the coding and modulationscheme, as well as for spatial processing. The channel estimate may bederived from a reference signal and/or channel condition feedbacktransmitted by the UE 650. Each spatial stream is then provided to adifferent antenna 620 via a separate transmitter 618TX. Each transmitter618TX modulates an RF carrier with a respective spatial stream fortransmission.

At the UE 650, each receiver 654RX receives a signal through itsrespective antenna 652. Each receiver 654RX recovers informationmodulated onto an RF carrier and provides the information to thereceiver (RX) processor 656. The RX processor 656 implements varioussignal processing functions of the L1 layer. The RX processor 656performs spatial processing on the information to recover any spatialstreams destined for the UE 650. If multiple spatial streams aredestined for the UE 650, they may be combined by the RX processor 656into a single OFDM symbol stream. The RX processor 656 then converts theOFDM symbol stream from the time-domain to the frequency domain using aFast Fourier Transform (FFT). The frequency domain signal comprises aseparate OFDM symbol stream for each subcarrier of the OFDM signal. Thesymbols on each subcarrier, and the reference signal, is recovered anddemodulated by determining the most likely signal constellation pointstransmitted by the eNB 610. These soft decisions may be based on channelestimates computed by the channel estimator 658. The soft decisions arethen decoded and de-interleaved to recover the data and control signalsthat were originally transmitted by the eNB 610 on the physical channel.The data and control signals are then provided to thecontroller/processor 659.

The controller/processor 659 implements the L2 layer. Thecontroller/processor can be associated with a memory 660 that storesprogram codes and data. The memory 660 may be referred to as acomputer-readable medium. In the UL, the control/processor 659 providesdemultiplexing between transport and logical channels, packetreassembly, deciphering, header decompression, control signal processingto recover upper layer packets from the core network. The upper layerpackets are then provided to a data sink 662, which represents all theprotocol layers above the L2 layer. Various control signals may also beprovided to the data sink 662 for L3 processing. Thecontroller/processor 659 is also responsible for error detection usingan acknowledgement (ACK) and/or negative acknowledgement (NACK) protocolto support HARQ operations.

In the UL, a data source 667 is used to provide upper layer packets tothe controller/processor 659. The data source 667 represents allprotocol layers above the L2 layer. Similar to the functionalitydescribed in connection with the DL transmission by the eNB 610, thecontroller/processor 659 implements the L2 layer for the user plane andthe control plane by providing header compression, ciphering, packetsegmentation and reordering, and multiplexing between logical andtransport channels based on radio resource allocations by the eNB 610.The controller/processor 659 is also responsible for HARQ operations,retransmission of lost packets, and signaling to the eNB 610.

Channel estimates derived by a channel estimator 658 from a referencesignal or feedback transmitted by the eNB 610 may be used by the TXprocessor 668 to select the appropriate coding and modulation schemes,and to facilitate spatial processing. The spatial streams generated bythe TX processor 668 are provided to different antenna 652 via separatetransmitters 654TX. Each transmitter 654TX modulates an RF carrier witha respective spatial stream for transmission.

The UL transmission is processed at the eNB 610 in a manner similar tothat described in connection with the receiver function at the UE 650.Each receiver 618RX receives a signal through its respective antenna620. Each receiver 618RX recovers information modulated onto an RFcarrier and provides the information to a RX processor 670. The RXprocessor 670 may implement the L1 layer.

The controller/processor 675 implements the L2 layer. Thecontroller/processor 675 can be associated with a memory 676 that storesprogram codes and data. The memory 676 may be referred to as acomputer-readable medium. In the UL, the control/processor 675 providesdemultiplexing between transport and logical channels, packetreassembly, deciphering, header decompression, control signal processingto recover upper layer packets from the UE 650. Upper layer packets fromthe controller/processor 675 may be provided to the core network. Thecontroller/processor 675 is also responsible for error detection usingan ACK and/or NACK protocol to support HARQ operations. Thecontrollers/processors 675, 659 may direct the operation at the eNB 610and the UE 650, respectively.

The controller/processor 675 and/or other processors and modules at theeNB 610 may perform or direct operations, for example, operations 1100in FIG. 11, operations 1500 in FIG. 15, operations 1600 in FIG. 16and/or other processes for the techniques described herein for S-RLFmanagement. The controller/processor 659 and/or other processors andmodules at the UE 650 may perform or direct operations, for example,operations 1400 in FIG. 14, and/or other processes for the techniquesdescribed herein S-RLF management. In certain aspects, one or more ofany of the components shown in FIG. 6 may be employed to perform exampleoperations 1100, 1400, 1500, 1600 and/or other processes for thetechniques described herein. The memories 660 and 676 may store data andprogram codes for the UE 650 and eNB 610 respectively, accessible andexecutable by one or more other components of the UE 650 and the eNB610.

Example Procedures for Managing Secondary eNB (SeNB) Radio Link Failure(S-RLF) in Dual Connectivity Scenarios

Presently, UEs receive data from one eNodeB. However, users on a celledge may experience high inter-cell interference which may limit thedata rates. Dual connectivity allows users to communicate with twoeNodeBs simultaneously. It works by sending and receiving data from thetwo eNodeBs in two totally separate streams when a UE is in range of twocell towers in two adjacent cells at the same time. The UE talks to thetwo towers simultaneously when it is on the edge of either towers'reach.

FIG. 7 illustrates a dual connectivity scenario 700 in accordance withcertain aspects of the present disclosure. As shown in FIG. 7, UE 720 iswithin the overlapping portion of the cell edges of cells 702 a and 702b served by eNBs 710 a and 710 b respectively. As shown UE 720 maycommunicate (uplink and downlink) with both eNBs simultaneously usingindependent data streams to each of the eNBs. By scheduling twoindependent data streams to the UE from the two different eNodeBs at thesame time, dual connectivity exploits uneven loading. This helps improvethe cell edge user experience while increasing network capacity. In oneexample, throughput data speeds for users at a cell edge may double. Inan aspect, one or more network controllers 730 may be configured tocontrol the eNBs 710 a and 710 b to implement the dual connectivityscenario 700.

Dual connectivity may have benefits in the cellular industry. In anaspect, a dual connectivity solution enables a UE to simultaneouslyconnect to two eNBs, a Master eNB (MeNB) and a secondary eNB (SeNB)which are not collocated. Thus, the different eNBs may use differentschedulers, etc. In an aspect, dual connectivity may involve two subsetsof serving cells including a Master Cell group (MCG) containing servingcells of the MeNB, and a Secondary Cell Group (SCG) containing servingcells of the SeNB. In general, the term “dual connectivity” may be usedto refer to an operation where a given UE consumes radio resourcesprovided by at least two different network points connected with anon-ideal backhaul.

FIG. 8 illustrates a dual connectivity scenario 800 including a macrocell and a small cell communicating with a UE, in accordance withcertain aspects of the present disclosure. As shown in FIG. 8, the UE810 may be dually connected to the macro cell 802 and the small cell804, and the eNBs may be connected via a non-ideal backhaul 820 and mayoperate on different carrier frequencies. In an aspect, the small cellmay include a pico cell, a femto cell, or another macro cell (e.g., withlower power capability).

In certain aspects, inter-node radio resource aggregation may beemployed in a dual connectivity scenario for improving per-userthroughput. This may be done by aggregating radio resources in more thanone eNB for user plane data transmission. For example, carrieraggregation may be employed where multiple LTE/component carriers areaggregated to serve a single unit of LTE Advanced UE. It is envisionedthat under dual connectivity a UE may benefit from carrier-aggregation(CA) gains in heterogeneous deployments (where a combination of macroand small cells are used). With carrier aggregation UEs may use spectrumof up to 20 MHz bandwidths up to a total of 100 MHz (5 componentcarriers) for transmission in each direction. As shown in FIG. 8,Carrier 1 of the macro cell 802 and Carrier 2 of the small cell 804 areaggregated to serve UE 810.

In certain aspects, two types of carrier aggregation (CA) methods may beemployed, continuous CA and non-continuous CA, as illustrated in FIGS. 9and 10 respectively. Continuous CA occurs when multiple availablecomponent carriers are adjacent to each other (FIG. 9). As shown in FIG.9, LTE carriers 1, 2 and 3 are adjacent to each other may be aggregatedfor communication between a UE and eNB. On the other hand,non-continuous CA occurs when multiple available component carriers areseparated along the frequency band (FIG. 10). As shown in FIG. 10, LTEcarriers 1, 2, and 3 are separated along the frequency band and may beaggregated for communication between a UE and eNB.

In certain aspects, a UE operating in a multicarrier system (e.g., usingcarrier aggregation) is configured to aggregate certain functions of themultiple carriers, such as control and feedback functions, on the samecarrier, which may be referred to as a “primary carrier.” The remainingcarriers that depend on the primary carrier for support are referred toas associated secondary carriers. For example, the UE may aggregatecontrol functions such as those provided by the optional dedicatedcontrol channel (DCH), the nonscheduled grants, a physical uplinkcontrol channel (PUCCH), and/or a physical downlink control channel(PDCCH).

FIG. 11 illustrates a method 1100 for controlling radio links in amultiple carrier wireless communication system by grouping physicalchannels, in accordance with certain aspects of the present disclosure.As shown, the method 1100 may include, at block 1102, aggregatingcontrol functions from at least two carriers onto one carrier to form aprimary carrier and one or more associated secondary carriers. Next atblock, 1104, communication links may be established for the primarycarrier and each secondary carrier. Then, communication may becontrolled based on the primary carrier in block 1106.

In certain aspects, dual connectivity does not introduce a significantchange to the C-plane (Control-plane) architecture from the UEperspective. For example, the RRC (Radio Resource Control) messagescontinue to be transmitted over the MeNB, and, from the UE point ofview, there is a single RRC entity.

On the other hand, for the U-plane (User-plane), two architectures maybe supported for dual connectivity. FIGS. 12 and 13 show two differentarchitectures that may be supported for the U-plane. In a firstarchitecture (shown in FIG. 12) a data radio bearer is eNB-specific, andmay be served by the MeNB or SeNB, but not both. When served by theSeNB, this type of bearer may be referred to as SCG bearer. As shown inFIG. 12, bearer 1230 may be served only by the MeNB 1210 and bearer 1240may only be served by SeNB 1220.

An alternative U-plane architecture (shown in FIG. 13) enables a bearerto be served by both eNBs. This type of bearer may be referred to as asplit bearer. As shown in FIG. 13, split bearer 1330 may be served bothby the MeNB 1310 and the SeNB 1320. In certain aspects, a bearerestablishes a “virtual” connection between two endpoints so that trafficcan be sent between them. The bearer acts as a pipeline between the twoendpoints.

In certain aspects, due to the distributed nature of the dualconnectivity deployment scenario (separate eNBs connected via anon-ideal backhaul) separate uplink control channels for both eNBs (MeNBand SeNB) is used to support distributed scheduling and independent MAC(Medium Access Control) operation across the eNBs. This is unlike CA(Carrier Aggregation) deployment, in which a single MAC/schedulingentity operates across all the carriers and a single uplink controlchannel is used.

In the current LTE specification, the Primary Cell (PCell of MeNB) isthe only cell carrying the uplink control channels, e.g., the PUCCH. Fordual connectivity, a special cell on the SeNB, Primary Secondary Cell(SpCell), is introduced in order to support the uplink control channelsfor the SeNB. Also, with dual connectivity, uplink control channels forboth the MeNB and the SeNB are used, one for each eNB. In certainaspects, the presence of an uplink control channel for the SeNBmotivates the use for an SCG Radio Link Monitoring (S-RLM) procedure.This procedure may be used by the UE to trigger SeNB Radio Link Failure(S-RLF or SCG RLF). The S-RLF procedure is useful, among other things,to trigger procedures that prevent a UE from jamming the uplink controlchannels when it loses radio connection to an SeNB. Another reason aspecial RLF procedure (e.g., S-RLF) may be used for the SeNB is that theMeNB may experience different channel conditions than the SeNB.

In certain aspects, unlike the legacy RLF procedure, the S-RLF does notnecessarily involve loss of RRC connection since RRC is over the MeNBand the connection to the MeNB may remain. Hence, certain C-Planeprocedures (such as RRC Connection Reestablishment) may not beapplicable under S-RLF.

Aspects of the present disclosure discuss several procedures involved inthe detection, indication and recovery from S-RLF.

In certain aspects, the S-RLM procedures like the RLM procedures are onthe special cell of the SeNB, the SpCell. The Spcell may carry the ULcontrol (in the PUCCH) for the SeNB. It may be noted that anysupplementary carriers of the SeNB may not have associated RLMprocedures.

In certain aspects, the S-RLF procedure may include S-RLF detection,S-RLF indication, and at least one of S-RLF recovery or SeNB releaseincluding bearer fallback upon S-RLF.

In S-RLF detection, the UE or SeNB may determine that the link to theSpCell has undergone RLF based on one or more criteria discussed below.In S-RLF indication, the UE or SeNB may indicate to the MeNB that theSpCell has undergone RLF. The indication may be sent to the MeNB overthe backhaul by the SeNB or over RRC or MAC messaging by the UE asdescribed below.

Once the S-RLF has been detected and indicated to the MeNB, there may betwo different approaches to manage the S-RLF. A first alternative mayinclude S-RLF recovery, where the UE may re-establish the SeNBconnection. In an aspect, the re-establishment may be configured by theMeNB or performed autonomously by the UE as described below. A secondalternative may include SeNB release, for example, in cases where theSeNB connection may not be re-established. According to thisalternative, the SeNB link is released by the MeNB. In this alternative,e.g., for a split bearer architecture shown in FIG. 13, bearer fallbackmay be implemented where the traffic may be served exclusively by theMeNB concurrently with the SeNB release. After the SeNB release, allbearers may be served by the MeNB as described below. In an aspect, evenif there is no explicit SeNB release upon S-RLF (i.e., the SeNB remainsconfigured), the split bearer (see FIG. 13) may continue to be served bythe MeNB.

FIG. 14 illustrates example operations 1400 performed, for example, by aUE, in accordance with certain aspects of the present disclosure.Operations 1400 may begin at 1402 by establishing communication with aMeNB and a SeNB. At 1404, the UE may detect an RLF of a connection withthe SeNB, for example, by detecting one or more of Random Access failureon the SeNB, measured radio link quality (e.g., downlink quality)degradation of a link with the SeNB below a threshold resulting in highcontrol channel error probability, RLC re-transmission failures forbearers served by the SeNB, RLF on the MeNB link, SeNB handover (orchange) failure, or detecting loss of synchronization with the SeNB. At1406, the UE may transmit an indication of the SCG RLF to the MeNB, inresponse to the detection, for example, by at least one of transmittingan S-RLF indication RRC message or MAC message, triggering andtransmitting a measurement report on radio link quality conditions inone or more cells of the SeNB, or triggering and transmitting a PDCP(packet data convergence protocol) status report. In an aspect, theS-RLF indication message contains a measurement report regarding one ormore cells of the SeNB. At 1408, the UE may continue to monitor,measure, and report the SeNB radio link quality. At 1410, the UE may,upon detection of improved radio conditions, perform a random accessprocedure on the SeNB. At 1412, the UE may re-establish connection withthe SeNB.

FIG. 15 illustrates example operations 1500 performed, for example, by aMeNB, in accordance with certain aspects of the present disclosure.Operations 1500 may begin at 1502 by establishing a first connectionwith a UE. At 1504, the MeNB may configure the UE to establish a secondconnection with a SeNB, for example, including configuring the UE toperiodically report SeNB measurements. In an aspect, the configuring mayinclude the MeNB sending configuration parameters for the S-RLF to theUE. At 1506, the MeNB may receive an indication of a RLF of the secondconnection with the SeNB, for example, including receiving a reportwhere the measured received signal for the SeNB is below a certainthreshold. At 1508, the MeNB may take at least one action to manage theRLF, in response to receiving the indication of the RLF. As noted above,the one or more actions may include S-RLF recovery or SeNB releaseincluding bearer fallback. In an aspect, the one or more actions mayinclude transmitting a reconfiguration command to the UE, including atleast one of SeNB release, SeNB add (e.g., another SeNB), SeNB modify(e.g., modify transmit power, data rate or other parameters), or datafallback. At 1510, the MeNB may perform data fallback if the SeNB isreleased.

FIG. 16 illustrates example operations 1600 performed, for example, by aSeNB, in accordance with certain aspects of the present disclosure.Operations 1600 may being at 1602 by establishing communication with aMeNB and a UE. At 1604, the SeNB may detect an RLF of a connection withthe UE, for example, by monitoring the UE's uplink transmissions for theSeNB and detecting when the received energy of the uplink transmissionsfall below a threshold. In an aspect, this detection may includemonitoring the UE's uplink transmissions for the SeNB and detecting whenthe received energy of the uplink transmissions falls below a threshold.At 1606, the SeNB may transmit an indication of the RLF to the MeNB overa backhaul connection, in response to the detection.

S-RLF Detection

In certain aspects, a UE connected to a SeNB may detect S-RLF in severalways including one or more of Random Access failure on SeNB, measuredradio link quality (e.g., downlink quality) degradation of a link withthe SeNB below a threshold resulting in high control channel errorprobability, RLC (Radio Link Control) retransmission failures, RLF onthe MeNB link, SeNB handover (or change) failure, or detecting loss ofsynchronization with the SeNB.

The SeNB may detect S-RLF for a connected UE by monitoring its uplinktransmissions (e.g., control channels, Sounding Reference Signal (SRS),etc.) and by determining that received energy (or signal-to-noise ratio)of the transmissions is below a minimum threshold level.

As an alternative example, the MeNB may detect S-RLF by configuring theUE to report SeNB measurements (e.g. periodically) and by receiving areport where the measured received signal for the SeNB is below acertain threshold.

In certain aspects, upon S-RLF detection the UE stops all uplinktransmissions and/or stops decoding on all downlink data and controlchannels.

S-RLF Indication

A UE connected to an SeNB and MeNB and undergoing S-RLF (but not RLF onMeNB) may indicate S-RLF to MeNB by at least one of transmitting anS-RLF indication RRC message or MAC message, triggering and transmittinga measurement report on radio link quality conditions in one or morecells of the SeNB, or triggering and transmitting a PDCP (Packet DataConvergence Protocol) status report. In an aspect, the SeNB, upondetection of S-RLF of a UE may indicate S-RLF to MeNB by transmittinginformation indicating S-RLF over the backhaul to the MeNB.

S-RLF Recovery—S-RLF Autonomous Recovery

In certain aspects, the UE may remain configured with SeNB upon S-RLFdetection, until de-configured by SeNB via SeNB release procedure. AfterS-RLF detection and while still configured with SeNB, the UEexperiencing S-RLF may continue to monitor the SeNB radio link qualityto potentially resume data transmission to the SeNB. Upon detection ofimproved radio condition (e.g., SeNB SNR>threshold) the UE in S-RLF mayperform a Random Access Procedure on the SeNB without being signaled bythe MeNB. Upon successful completion of the Random Access on SeNB, theUE may re-establish connection with the SeNB and may indicate S-RLFrecovery to the MeNB via an RRC or MAC message.

S-RLF Recovery—S-RLF Network-Assisted Recovery

As noted above, the UE may remain configured with SeNB upon S-RLFdetection, until de-configured by SeNB via SeNB release procedure. AfterS-RLF detection and while still configured with SeNB, the UEexperiencing S-RLF may continue to measure and report, e.g.,periodically or at configured times, the SeNB radio link quality to theMeNB. Upon reception of a report from the UE indicating suitable radiolink quality between the UE and the SeNB, the MeNB may signal the UE toperform a Random Access Procedure on the SeNB to re-establish connectionwith the SeNB.

In an aspect, after detecting the S-RLF, the UE may continue to monitorand measure the radio link quality with the SeNB. Upon detection of arecovery of the radio link with the SeNB, for example upon detecting thequality above a threshold quality, the UE may transmit an indication ofthe recovery to the MeNB. In response, the MeNB may transmit a S-RLFrecovery command to the UE including instructions for re-establishingconnection with the SeNB. For example, the S-RLF recovery command mayinclude instructions to perform Random Access with the SeNB. In anaspect, the indication of recovery transmitted to the MeNB may include aS-RLF recovery message or a measurement report regarding the SeNB.

SeNB Release and Bearer Fallback Upon S-RLF

In certain aspects, upon receiving an S-RLF indication, the MeNB mayinitiate a SeNB release procedure with the UE in which the bearersconnected to the SeNB are removed. In an aspect, concurrent with therelease procedure, the MeNB may perform a data bearer fallback,consisting of resuming data communication on MeNB only, for at least onebearer (or data flow) that is served by MeNB and SeNB simultaneously. Inan aspect, data bearer fallback may not require reconfiguration of PDCP.This fallback may be done because in the alternative U-planearchitecture a bearer may be served by both eNBs (see FIG. 13).

It is understood that the specific order or hierarchy of steps in theprocesses disclosed is an illustration of exemplary approaches. Basedupon design preferences, it is understood that the specific order orhierarchy of steps in the processes may be rearranged. The accompanyingmethod claims present elements of the various steps in a sample order,and are not meant to be limited to the specific order or hierarchypresented.

The previous description is provided to enable any person skilled in theart to practice the various aspects described herein. Variousmodifications to these aspects will be readily apparent to those skilledin the art, and the generic principles defined herein may be applied toother aspects. Thus, the claims are not intended to be limited to theaspects shown herein, but is to be accorded the full scope consistentwith the language claims, wherein reference to an element in thesingular is not intended to mean “one and only one” unless specificallyso stated, but rather “one or more.” Unless specifically statedotherwise, the term “some” refers to one or more. All structural andfunctional equivalents to the elements of the various aspects describedthroughout this disclosure that are known or later come to be known tothose of ordinary skill in the art are expressly incorporated herein byreference and are intended to be encompassed by the claims. Moreover,nothing disclosed herein is intended to be dedicated to the publicregardless of whether such disclosure is explicitly recited in theclaims. No claim element is to be construed under the provisions of 35U.S.C. § 112, sixth paragraph, unless the element is expressly recitedusing the phrase “means for” or, in the case of a method claim, theelement is recited using the phrase “step for.”

What is claimed is:
 1. A method for wireless communication by a userequipment (UE), comprising: establishing dual-connectivity communicationwith a master base station and a secondary base station; detecting radiolink failure (RLF) of a connection with the secondary base station;transmitting an indication of the RLF to the master base station; andreceiving a reconfiguration command from the master base station aftertransmitting the indication of the RLF, wherein the reconfigurationcommand comprises a command for data fallback including resuming datacommunication with the master base station on at least one bearerpreviously established with the secondary base station andsimultaneously served by both the master base station and the secondarybase station, wherein resuming data communication with the master basestation for the data fallback is performed without a PDCP (Packet DataConvergence Protocol) re-configuration of the at least one bearer. 2.The method of claim 1, further comprising: releasing one or more bearersserved only by the secondary base station in response to a communicationby the master base station.
 3. The method of claim 1, wherein the atleast one bearer comprises a split-bearer and wherein the split-bearercontinues to be served by the master base station following the RLF ofthe connection with the secondary base station.
 4. The method of claim1, wherein transmitting the indication of the RLF comprises transmittingat least one of a Secondary-RLF (S-RLF) indication message, ortransmitting a measurement report regarding one or more cells of thesecondary base station.
 5. The method of claim 1, wherein the UE remainsconfigured with the secondary base station in the dual-connectivitycommunication until de-configured.
 6. The method of claim 1, whereindetecting RLF of the connection with the secondary base stationcomprises at least one of: detecting a random access failure on thesecondary base station, detecting a degradation of radio link qualitywith the secondary base station, detecting one or more RLCretransmission failures with the secondary base station, detecting aloss of synchronization with the secondary base station, or detecting ahandover failure with the secondary base station.
 7. The method of claim1, wherein transmitting the indication of the RLF comprises transmittinga radio resource control (RRC) message or a medium access control (MAC)message.
 8. The method of claim 1, wherein transmitting the indicationof the RLF comprises transmitting a measurement report with informationabout a radio link quality of the connection with the secondary basestation.
 9. The method of claim 1, further comprising suspendingcommunication with the secondary base station in response to detectingthe RLF.
 10. The method of claim 1, further comprising: continuing tomonitor radio link quality with the secondary base station afterdetecting the RLF; and detecting recovery of a radio link with thesecondary base station.
 11. The method of claim 10, wherein detectingrecovery of the radio link comprises determining that a measureddownlink signal from the secondary base station exceeds a thresholdvalue.
 12. The method of claim 10, further comprising: re-establishingcommunication with the secondary base station, in response to detectingthe recovery of the radio link, by performing a random access procedure.13. A method of wireless communication performed by a master basestation, comprising: establishing dual-connectivity communication with auser equipment (UE) and a secondary base station; receiving, from theUE, an indication of a radio link failure with the secondary basestation; and transmitting, to the UE, a reconfiguration commandcomprising a command for data fallback including resuming datacommunication with the master base station on at least one bearerpreviously established with the secondary base station andsimultaneously served by the master base station and the secondary basestation, wherein resuming data communication with the master basestation for the data fallback is performed without a PDCP (Packet DataConvergence Protocol) re-configuration of the at least one bearer. 14.The method of claim 13, wherein the reconfiguration command comprises acommand to release one or more bearers previously established with thesecondary base station.
 15. The method of claim 13, wherein the at leastone bearer comprises a split-bearer and wherein the split-bearercontinues to be served by the master base station following the RLF ofthe connection with the secondary base station.
 16. The method of claim13, wherein the indication of the RLF comprises at least one of aSecondary-RLF (S-RLF) indication message, or a measurement reportregarding one or more cells of the secondary base station.
 17. Themethod of claim 13, wherein the indication of the RLF comprises a radioresource control (RRC) message or a medium access control (MAC) message.18. The method of claim 13, wherein the indication of the RLF comprisesa measurement report with information about a radio link quality of aconnection with the secondary base station.
 19. The method of claim 13,further comprising: transmitting a command to the UE includinginstructions for reestablishing communication with the secondary basestation.
 20. The method of claim 19, wherein the instructions forreestablishing communication with the secondary base station instructthe UE to perform a random access procedure with the secondary basestation.
 21. An apparatus for wireless communication, comprising: atleast one processor configured to: establish dual-connectivitycommunication with a master base station and a secondary base station;detect radio link failure (RLF) of a connection with the secondary basestation; transmit an indication of the RLF to the master base station;and receive a reconfiguration command from the master base station aftertransmitting the indication of the RLF, wherein the reconfigurationcommand comprises a command for data fallback including resuming datacommunication with the master base station on at least one bearerpreviously established with the secondary base station andsimultaneously served by both the master base station and the secondarybase station, wherein resuming data communication with the master basestation for the data fallback is performed without a PDCP (Packet DataConvergence Protocol) re-configuration of the at least one bearer; and amemory coupled to the at least one processor.
 22. The apparatus of claim21, wherein the at least one bearer comprises a split-bearer and whereinthe split-bearer continues to be served by the master base stationfollowing the RLF of the connection with the secondary base station. 23.The apparatus of claim 21, wherein transmitting the indication of theRLF comprises transmitting at least one of a Secondary-RLF (S-RLF)indication message, or transmitting a measurement report regarding oneor more cells of the secondary base station.
 24. The apparatus of claim21, wherein transmitting the indication of the RLF comprisestransmitting a radio resource control (RRC) message or a medium accesscontrol (MAC) message.
 25. The apparatus of claim 21, wherein the atleast one processor is further configured to suspend communication withthe secondary base station in response to detecting the RLF.
 26. Theapparatus of claim 21, wherein the at least one processor is furtherconfigured to: continue to monitor radio link quality with the secondarybase station after detecting the RLF; and detect recovery of a radiolink with the secondary base station.
 27. An apparatus for wirelesscommunication, comprising: at least one processor configured to:establish dual-connectivity communication with a user equipment (UE) anda secondary base station; receive, from the UE, an indication of a radiolink failure with the secondary base station; transmit, to the UE, areconfiguration command comprising a command for data fallback includingresuming data communication with the apparatus on at least one bearerpreviously established with the secondary base station andsimultaneously served by the apparatus and the secondary base station,wherein resuming data communication for the data fallback is performedwithout a PDCP (Packet Data Convergence Protocol) re-configuration ofthe at least one bearer; and a memory coupled to the at least oneprocessor.
 28. The apparatus of claim 27, wherein the reconfigurationcommand comprises a command to release one or more bearers previouslyestablished with the secondary base station.
 29. The apparatus of claim27, wherein the at least one bearer comprises a split-bearer and whereinthe split-bearer continues to be served by the apparatus following theRLF of the connection with the secondary base station.
 30. The apparatusof claim 27, wherein the indication of the RLF comprises at least one ofa Secondary-RLF (S-RLF) indication message, or a measurement reportregarding one or more cells of the secondary base station.